Uterine leiomyomas, also known as uterine fibroids, are benign, fibrotic smooth muscle tumors that affect an alarming 75-80% of women in their lifetime and constitute a major health problem worldwide. They remain shockingly understudied with few nonsurgical therapeutic options. In a major breakthrough in the field, mutations in the transcriptional mediator complex subunit 12, termed Med 12, have been identified at very high frequency in leiomyoma implying that Med12 mutations are the driver mutations. However, how Med12 and its mutations contribute to leiomyoma is completely unknown. The long term goal of our laboratory is to discover novel molecular events driving leiomyoma. The immediate goal of this R21-exploratory application is to test the hypothesis that Med12 mutations function by generating an altered Med12/mediator cistrome in leiomyoma, leading to abnormal expression of cell proliferation/apoptosis and profibrotic genes. We will determine the genome wide binding profile of wild type and mutant Med12 and other key mediator subunits in normal myometrial and leiomyoma primary cells. Using bioinformatic and ChIP qPCR analysis we will determine whether Med12 functions primarily by targeting communication between transcription factors such as nuclear receptors and Smad proteins, mediator complex and RNA polymerase II and establish that such interactions are defective/altered in leiomyoma. Together, these results will determine for the first time molecular defects in Med12 in leiomyoma thereby advancing the field in an unprecedented manner. Additionally, the proposed exploratory analyses will generate new hypothesis for future RO1-type research proposals in this highly understudied human health problem. The proposed work is scientifically, translationally, and clinically significant because i will provide a new perspective on the key driver mechanisms of Med 12 function in leiomyoma development and open up possibilities for identifying additional therapeutic targets and strategies to treat this disease. It is innovative because it represents the first systematic exploration of previously unrealized roles of Med12 in leiomyoma patho-biology.